Various smoke detectors include a chamber through which a sample of air is drawn and studied to determine if particles are present. Over time the accumulation of dust and debris on surfaces within the detection chamber can affect the operation of the detector.
By way of example, scattering light detectors include a light source arranged to project a beam across the detection chamber. A photoelectric sensor is arranged so that its field of view is traversed by a portion of the beam. The photoelectric sensor receives light scattered from the beam due to the presence of particles in the detection chamber. Over time dust and debris can accumulate on surfaces within the detection chamber and reflect light towards the photoelectric sensor thereby providing a false indication of particles in the detection chamber. Dust and debris may also settle on the light source and/or the photoelectric sensor thereby obscuring the transmission and receipt of light and reducing the sensitivity of the detector.
One approach to addressing these problems involves the use of an ‘air barrier’. An air barrier is created by directing one or more streams of clean air into the detection chamber to flow over the critical components, such as the light source, the photoelectric sensor and walls within the field of view of the photoelectric sensor, to prevent dust and debris accumulating thereon.
Aspirated smoke detectors employ a fan, known as an aspirator, to draw air to be studied through the detection chamber. The air to be studied enters the chamber via an inlet(s). A desirable implementation of the air barrier concept employs a filter(s) to create the clean air. The filter is arranged in parallel to the inlet, whereby the clean air is drawn through the filter and into the detection chamber by the aspirator. A common stream of air, e.g. from a network of pipes, may be divided into two portions—one portion being filtered to create the clean air, and the other portion entering the chamber to be studied.
Another approach to addressing the problems associated with the accumulation of dust and debris in the detection chamber is to obtain a measurement associated with light reflected from the accumulated dust and debris, known as ‘background light’, and to adjust the detection criteria applied to the signal received from the photoelectric sensor in response to the background light. One approach to obtaining a measure of background light involves the use of a second photoelectric sensor within the detection chamber. The second photoelectric sensor is arranged so that its field of view does not include the beam. Signals from the second photoelectric sensor are thereby indicative of light reflected within the detection chamber rather than light scattered directly from the beam.
The abstract of Japanese patent application 59192940 is entitled Smoke Meter with Purging Device and describes filling a measuring device with clean air and measuring opaqueness in the clean atmosphere to perform calibration. The described device includes a dedicated blower to supply clean air to the detection chamber. A valve controlled by depressible switches is used to close the intake tube to halt the flow of discharge gases to the detection chamber prior to the purging operation.
New Zealand patent 250497 is concerned with preventing fire suppression measures being activated in response to false alarms. It describes an operating syntax applicable to aspirated smoke detectors. When an alarm condition is detected the chamber is purged with clean air and a background ‘smoke’ signal is measured. If the background reading does not fall below a predetermined threshold a detector fault is indicated. If the background ‘smoke’ falls below the predetermined threshold, the system waits for the detected smoke level to rise above a further threshold before triggering the fire suppression systems.
It is an object of the invention to provide an improved particle detector.